The flame retardancy and mechanical properties of waste polyester fabric/hemp stalk composites modified with piperazine pyrophosphate and ammonium polyphosphate

The piperazine pyrophosphate (PAPP) and ammonium polyphosphate (APP) were employed to enhance the flame retardancy of waste polyester/hemp stalk composites. And the impact of PAPP/APP flame retardants on the flame retardancy and mechanical properties of waste polyester/hemp stalk was investigated. The results indicated that the PAPP/APP combination offered superior flame retardancy, with minimal impact on the composites’ bending property. Notably, when the total addition of PAPP/APP was 25%, with a mass ratio of 1 to 1, the limiting oxygen index (LOI) value of the PAPP/APP/waste polyester/hemp stalk composite material increased significantly from 18.8% to 27.1%, achieving the flame retardant level. Correspondingly, the vertical combustion level attained V-0 level, and the charring rate at 600°C escalated from the initial 25.8% to 32.5%. Remarkably, both the bending strength and modulus of PAPP/APP/waste polyester/hemp stalk composites remained stable.


Introduction
Associated with the large volume of waste polyester textiles, it is difficult to degrade and recycle effectively [1,2], as well as the environmental pollution caused by the burning of hemp stalks [3,4].It was proposed that waste polyester fabrics can be used as the base material, discarded hemp stalks as the reinforcing material, and waterborne epoxy resin adhesive as the binder.This kind of composite material is prepared through a process of physical blending and molding.Not only does this approach solve the problem of waste disposal, but it also provides a viable alternative to wood in various applications such as construction materials, furniture, interior and exterior decoration, and automotive parts, thereby transforming "waste into wealth".However, both waste polyester and hemp stalk powder are flammable materials.Their combustion produces large amounts of CO, CO2, dense smoke, and heat, and the burning of polyester fabric can result in the formation of molten droplets.Once ignited, it is difficult to extinguish and can rapidly spread, igniting surrounding materials and causing irreparable major damage [5][6][7].This makes the flame retardancy of the waste polyester fabric/hemp stalk composite material substandard, severely hindering its application in areas such as automotive interiors, exterior building walls, or decorative wall panels.To meet the relevant product standards, it is necessary to modify its flame retardancy.Intumescent flame retardants (IFR) are a type of environmentally friendly retardant that expand and foam during combustion to provide flame resistance [8][9][10].Typically, IFRs comprise three components: an acid source, a carbon source, and a gas source.When the temperature reaches a certain threshold, the 2 acid source decomposes first, prompting the carbon source to dehydrate into carbon, thereby activating the flame retardant [11,12].Throughout the combustion process, the IFR undergoes a series of chemical reactions, including esterification, carbonization, expansion, and curing, resulting in a porous expanded carbon layer that serves as a barrier to heat and oxygen [9-10'].Ammonium polyphosphate (APP), acting as an acid and gas source in a halogen-free system, can promote the formation of an expanded carbon layer in the condensed phase during combustion, isolating oxygen.In the gas phase, it generates gases such as ammonia and steam to dilute the concentration of combustible gases, thereby slowing down the combustion process [13,14].Pyrophosphoric acid piperazine (PAPP) exhibits good water resistance and carbon-forming properties, and contains an acid source, carbon source, and gas source.It can function as a single-component IFR, providing flame resistance in both the gas and condensed phases [15][16][17].Therefore, this study discussed different ratios of PAPP/APP as composite flame retardant system to modify the waste polyester/hemp stalk flame-retardant composite materials.By analyzing the flame retardancy, mechanical properties, and thermal stability, the waste polyester/hemp stalk composites with excellent comprehensive performance was prepared.

Experimental material
Waste polyester fabrics were obtained from a textile recycling factory in Taiyuan; discarded hemp stalks were sourced from a hemp base in Jinzhong; pyrophosphoric acid piperazine (PAPP) was provided by GaoFeng Polymer Materials.Ammonium polyphosphate (APP) was supplied by Yunnan Tianyao.γamino-propyltriethoxysilane was procured from Nanjing Chen Gong Organic Silicon Materials Co., Ltd..The waterborne epoxy resin emulsion (viscosity>2000MPA•S) and waterborne epoxy curing agent (viscosity >2000MPA•S) were both purchased from Shenzhen Yoshida Chemical Co., Ltd.And sodium hydroxide (granular) was acquired from Tianjin Beichen Fangzheng Reagent Factory.

Experimental apparatus
The VT-LH20A flat vulcanizing machine was provided by Dongguan Instrument Detection Equipment Technology Co., Ltd.; the DA2004N electronic balance was supplied by Shanghai Jinghai Instrument Co. Ltd..The DHG-9075AD electric thermostatic blast drying oven was procured from Shanghai Boxun Industrial Co., Ltd.; the DLH-1000A high-speed multifunctional pulverizer was obtained from Wuyi Qiteng Instrument Company.The CMT4204G microcomputer-controlled high-temperature electronic universal testing machine was purchased from Shandong Shenglin Precision Machinery Equipment Co., Ltd..The M606B digital oxygen index detector was acquired from Qingdao Shanfang Instrument Co., Ltd..The CZF-3 horizontal and vertical combustion detector was provided by Nanjing Jiangning Analytical Instrument Co., Ltd..And the TGA-4000 thermogravimetric analyzer was supplied by the American company, PerkinElmer.

Preparation of composite materials
(1) Pre-treatment of waste polyester fabric: the recycled waste polyester fabric was sterilized at high temperature and then densified into polyester granules.After cleaning and drying, they were crushed with a pulverizer, and polyester powder with 30 mesh was obtained through a standard sieve with a pore size of 0.6 mm.
(2) Pre-treatment of waste hemp stalk: first of all, the waste hemp stalk was folded into small strips of 1-2 cm, washed and dried with distilled water, then soaked in 8% sodium hydroxide solution for 4 h, washed with distilled water until the filtrate was neutral, dried and crushed with a pulverizer, and hemp stalk powder with 50 mesh was obtained through a standard sieve with a pore size of 0.35 mm.
(3) Preparation of waterborne epoxy resin glue: waterborne epoxy resin emulsion, waterborne epoxy curing agent and γ-amino-propyltriethoxysilane (KH550) with the mass ratio of 8:4:3 (total mass of 15 g) fully mixed to prepare waterborne epoxy resin glue at room temperature.(4) Setting ratio of flame retardant in the matrix: the specific ratio of the flame retardant was shown in Table 1.12.5 12.5 Note: The quality ratio of PAPP to APP is 1:1; The mass fraction of PAPP or APP is the mass percentage of matrix material.
(5) Compression Molding: The pre-treated polyester powder and hemp stalk powder (with a constant total mass of 25g) were mixed with flame retardant in a predetermined ratio.This mixture was then thoroughly stirred with a prepared aqueous epoxy resin solution in a beaker before being manually cast into an iron mold.The mold was then dried in an oven at 80°C for 90 minutes.The dried composite preform was placed in a flat plate vulcanizing machine, with the processing temperature of 180°C, the pressure of 10MPa, and the hot-pressing time of 15 minutes.Finally, the composite material was obtained after cooling.

Test and characterization
(1)The limiting oxygen Index (LOI): According to GB/T 2406.1-2009standard, the sample size is 80 mm×10 mm×3.3 mm , and the result is the average of five samples.
(2)UL94 Vertical Flame Test: According to ASTM D380l-2010 standard, sample size is 125 mm × 13 mm × 3 mm, and the average value of five groups is taken.Thermogravimetric analysis (TG): In nitrogen atmosphere, the test temperature range is 30-650 ℃, the heating rate is 10 ℃/min.
(3)Mechanical property test: According to GB/T 9341-2008 standard, by the 3-point bending method, the test condition is with the sample size of 60 mm × 25 mm × 3 mm, a span of 48 mm, and the loading speed of 10 mm/min.The result is the average data of five sample groups.(4)Microstructure characterization: The fracture surface of waste polyester/hemp stalk composites before and after flame retardant modification were sprayed with gold before test, and observed using a JSM-IT700HR field emission scanning electron microscope with an accelerating voltage of 5 kV.

Characterization of flame retardancy
Table 2 summarized the Limiting Oxygen Index (LOI) and vertical burning grade results for various waste polyester/hemp stalk composite materials.From the LOI test, it can be seen that the LOI value of the waste polyester/hemp stalk composites was only 18.8%, which was extremely flammable and had no vertical burning rating.Upon individual comparison of specimens 1-5 with specimens 6-9, it was observed that the LOI index of modified composites with solely PAPP flame retardant exceeded that of composites with only APP flame retardant.This suggested a superior flame retardant effect of PAPP over APP.This could be attributed to PAPP's decomposition during combustion, which led to the generation of substances such as polyphosphoric acid and metaphosphoric acid.These substances facilitated the formation of a continuous, dense carbon layer within the composite matrix, which was found to contain elevated levels of P and C elements.For APP/waste polyester/hemp stalk composites, its catalytic carbon-forming ability was found to be insufficient, resulting in a relatively low content of the C element [18][19][20], which led to a slightly inferior flame retardant effect, compared to PAPP/waste polyester/hemp stalk composites.However, it was observed from the table that the LOI index of PAPP/APP/waste polyester/hemp stalk composites increased continuously with the rise in the additive amount of PAPP/APP.When the additive amount was 25%, the LOI index of PAPP/APP/waste polyester/hemp stalk composites reached 27.1% and the vertical combustion grade achieved a V-0 grade.Thus, when the addition amount was 25%, the LOI index could reach 27.1% and the vertical combustion grade passed the V-0 level.In comparison to the single addition of 25% PAPP or APP, the LOI index was only 23.6% and 22.5%, and the vertical combustion grade of the composites did not achieve the V-0 grade.These proved that the PAPP/APP was more effective in enhancing the flame retardant performance of the waste polyester/hemp stalk composites.This demonstrated that PAPP and APP exhibited significant synergistic effects in the modification of waste polyester/hemp stalk.PAPP12.5/APP212.527.1 V-0 Note: The quality ratio of PAPP to APP is 1:1

Analysis of thermal stability
For polymer matrix composites, the charring yield not only has a great impact on the oxygen consumption and volatiles generation required during the combustion process, but also the formation of the char layer hinders the thermal degradation of the composite in the next step of combustion [21,22].To investigate the char formation at high temperature is important to characterize the flame retardant properties of composites by using thermogravimetric analyzer.Figure 1 showed the TG and DTG curves of different composites.Table 3 listed the initial weight loss temperature T5%, the maximum weight loss temperature T max, and the charring rate at 600°C of the composites with different flame retardant modifications.In conjunction with Figure 1 and Table 3, it was observed that the heat loss curve of waste polyester/hemp stalk composites noticeably decreased, while the heat loss curve of modified waste polyester/hemp stalk composites slightly flattened.This suggested that the thermal stability of the former was inferior, and the addition of flame retardant enhanced the thermal stability performance of waste polyester/hemp stalk composites to a certain extent.At the same time, the addition of flame retardants also sequentially reduced the initial weight loss temperature of the waste polyester/hemp stalk composites, and elevated the maximum weight loss temperature of the waste polyester/hemp stalk composites, which was caused by the thermal decomposition of flame retardants in the early stage.The decomposition of the flame retardant absorbed heat at the same time, thus reducing the beginning pyrolysis temperature of the composite material, delaying the pyrolysis process of the waste polyester/hemp stalk composite material, and increasing the temperature of the waste polyester/hemp stalk composite material at the maximum pyrolysis weight loss [23].The charring rate of waste polyester/hemp stalk composites at 600°C was only 25.8%, and the charring rates of composites with PAPP or APP alone have increased by 10.1% and 14.7%, respectively.while the charring rate of composites with the addition of PAPP and APP combined flame retardant was increased by 26.0% to 32.5%.The reasons for the aforementioned results were twofold: on one hand, the mass of decomposition products of PAPP or APP at high temperature exceeded the amount of residue at high temperature of composites of the same mass.On the other hand, the concurrent addition of PAPP and APP flame retardants exhibited a synergistic effect, leading to a catalytic acceleration of waste polyester/hemp stalk composites during pyrolysis.Compared to the addition of a single flame retardant, PAPP and APP could synergistically promote the formation of a continuous dense char layer, resulting in a significant increase in quality retention.As seen in Figure 1b, the mass loss rate of waste polyester/hemp stalk composites was the largest, and the mass loss rate of PAPP/APP/waste polyester/hemp stalk composites was the smallest.This was consistent with the results from the TG graph of the composites.Figure 2 depicted the impact of flame retardant modification on the flexural properties of waste polyester/hemp stalk composites.As seen in the figure 2, when the mass fraction of PAPP or APP was 25 wt.%, it had a lesser effect on the flexural properties of the waste polyester/hemp stalk composite material compared to the single flame retardant modification.This might have been due to the fact that a single addition of PAPP disrupted the homogeneous phase in the waste polyester/hemp stalk composite material, causing the flame retardant to distribute within it and create a phenomenon akin to the "seaisland effect".When the material was subjected to external forces, the flame retardant did not move along with the continuous phase, resulting in fracture and a decrease in bending properties.Compared to PAPP flame retardant, APP particles in the waste polyester/hemp stalk composite material was also prone to agglomeration, leading to stress concentration at the interfacial bonding, thereby damaging the interfacial bonding of hemp culm powder and polyester powder, and reducing the bending strength.However, formulating APP and PAPP with different particle sizes in a 1:1 ratio not only increased the surface area of the particles to some extent, but also had the effect of blocking the expansion of silver grain, and simultaneously, facilitated better dispersion in the matrix material.Therefore, the compounded flame retardant system exhibited higher mechanical properties [24,25].As seen in Figure 2a, the bending strength exhibited a trend of increasing and then decreasing with the rise in the additive amount of the PAPP/APP compound flame retardant.The bending strength was at its peak, 39.2 MPa, when the additive amount of PAPP/APP was 15%.This was due to the fact that both APP and PAPP flame retardants were oligomers, which played a certain plasticizing role in the waste polyester/hemp stalk composites preparation, thereby improving the bending strength of flame-retarded composite materials compared to non-flame-retarded composites.However, as the flame retardant content increased, the flexural strength decreased.This could be attributed to the fact that excessive flame retardant loading might disrupt the continuity of the material due to a malignant effect, and it was likely to cause stress concentration during the bending process, thereby reducing the mechanical properties of the waste polyester/hemp stalk composites [26].Furthermore, compared to the flexural modulus of waste polyester/hemp stalk composites modified with a single flame retardant, the use of the PAPP/APP compounded flame retardant had a lesser impact on the flexural modulus of the waste polyester/hemp stalk composites.This could be attributed to the fact that as the amount of PAPP and APP flame retardants increased, these two flame retardants acted as rigid fillers to compensate for the decrease in flexural modulus due to certain interfacial defects.Therefore, compared to the single PAPP or APP flame retardant modification, When the total addition amount of PAPP/APP compound flame retardant was 25%, the flexural strength and elastic modulus of the waste polyester/hemp stalk composites remained basically unchanged.12.5%APP+12.5%PAPP.Figure 3 displayed the fracture surface of the waste polyester/hemp stalk composites both prior to and following flame retardant modification.It was observed that, in contrast to the waste polyester/hemp stalk composites lacking flame retardant, the fracture cross-section of the waste polyester/hemp stalk composites with solely added flame retardant was notably rough, with an increased number of holes on the rough surface.Furthermore, there were clear signs of fiber pull-out in certain areas, a consequence of the flame retardant's disruption of the homogeneous phase within the waste polyester/hemp stalk composites, leading to some fibers agglomeration.The addition of PAPP or APP flame retardant disrupted the homogeneous phase in the waste polyester/hemp stalk composite material, resulting in partial agglomeration of the fibers.The bending cross-section of the waste polyester/hemp stalk composite material, modified with a 1:1 compounding of PAPP and APP, was relatively smooth.This suggested that the PAPP/APP compound flame retardant's dispersion in the matrix of waste polyester/hemp stalk composites was excellent, with no evident agglomeration among the PAPP/APP/waste polyester/hemp stalk composite materials.This observation was consistent with the results of the mechanical properties, and it also indicated that the interfacial compatibility of the waste polyester/hemp stalk composite material was superior under the PAPP/APP compound flame retardant.

Conclusion
1.In comparison to the singular addition of either APP or PAPP flame retardants, the compounding of PAPP and APP in waste polyester/hemp stalk composites demonstrates superior flame retardant properties.When the mass ratio of PAPP to APP is 1:1 and the total additive amount is 25% , the PAPP/APP/waste polyester/hemp stalk composite material passes the V-0 grade of UL94, achieving the flame retardant grade.Concurrently, the thermal stability and charring rate of the waste polyester/hemp stalk composites are significantly improved, with the residual charring rate at 600℃ reaching as high as 32.5%.These findings indicate that the PAPP/APP flame retardant system is the most effective one for the fabrication of high-performance flame retardant composites.
2. When PAPP or APP flame retardant is individually incorporated, there is a reduction in the flexural strength and elastic modulus of the waste polyester/hemp stalk composites compared to those without any flame retardant.However, the addition of a combination of PAPP and APP essentially maintains the flexural strength and modulus of the waste polyester/hemp stalk composites.

Figure 2 .
Figure 2. Bending strength and bending modulus of waste polyester/hemp stalk composites before and after modification; (a) bending strength, (b) bending modulus.

Figure 3 .
Figure 3. Bending fracture SEM images of waste polyester/hemp stalk composites before and after flame retardant modification： (a) no flame retardants are added; (b) 25% PAPP; (c) 25%APP; (d)12.5%APP+12.5%PAPP.Figure3displayed the fracture surface of the waste polyester/hemp stalk composites both prior to and following flame retardant modification.It was observed that, in contrast to the waste polyester/hemp stalk composites lacking flame retardant, the fracture cross-section of the waste polyester/hemp stalk composites with solely added flame retardant was notably rough, with an increased number of holes on the rough surface.Furthermore, there were clear signs of fiber pull-out in certain areas, a consequence of the flame retardant's disruption of the homogeneous phase within the waste polyester/hemp stalk composites, leading to some fibers agglomeration.The addition of PAPP or APP flame retardant disrupted the homogeneous phase in the waste polyester/hemp stalk composite material, resulting in partial agglomeration of the fibers.The bending cross-section of the waste polyester/hemp stalk composite material, modified with a 1:1 compounding of PAPP and APP, was relatively smooth.This suggested that the PAPP/APP compound flame retardant's dispersion in the matrix of waste polyester/hemp stalk composites was excellent, with no evident agglomeration among the PAPP/APP/waste polyester/hemp stalk composite materials.This observation was consistent with the results of the mechanical properties, and it also indicated that the interfacial compatibility of the waste polyester/hemp stalk composite material was superior under the PAPP/APP compound flame retardant.

Table 1 .
Specific component of various flame retardant systems

Table 2 .
The LOI value and UL94 Level of the modified waste polyester/hemp stalk composites.

Table 3 .
Thermogravimetric data of flame retardant composites